Multi-layer heating element

ABSTRACT

A heating element and, in particular, a ceramic heating element, such as ceramic heating elements used in high temperature glow plugs for diesel engines and gas igniters. The heating element includes an electrical insulator and an electrically conductive layer. The conductive layer is formed from a single material and single composition. The method of manufacture includes the steps of forming the insulative layer and molding a conductive layer around the insulative layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 60/785,334, filed Mar. 23, 2006 which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a heating element and, in particular, a ceramic heating element, such as ceramic heating elements used in high temperature glow plugs for diesel engines and gas igniters, and methods of manufacture therefore.

Ceramic heating elements, such as the glow plug illustrated in FIG. 1, are well known in the industry. As illustrated in FIG. 1, a glow plug 2 typically includes a heating element having an electrically conducting core 8 surrounded by an electrically insulative layer 6. The insulative layer 6 is in turn surrounded by an outer resistive layer 4 which makes contact with the conducting core 8 at the electrical connection area 9. To manufacture the glow plug 2, illustrated in FIG. 1, the layered structure is formed by sequentially slip casting layers from suspensions of different compositions in a porous mold, and then sintered to form a monolithic body. The resulting body is then electrically connected to form a ceramic heating element.

One problem with sequential casting is that the geometric configuration of the heating element is generally limited to shapes that allow each progressive layer to be formed against the previous layer. In the case of slip casting, the configuration of any layer is generally limited to a thin layer of fairly uniform thickness or a core that is substantially solid, but may be partially hollow due to piping which occurs as the cast material solidifies. This sequential stacking of layers limits the geometric configuration and prevents each layer from being optimized for use in a heating element and being optimized for use in particular applications.

Another drawback to the style of glow plugs 2 illustrated in FIG. 1 is that the sequential layering creates discrete interfaces between layers and when the glow plug is cycled between cold and hot temperatures, failures may occur. To reduce the failure rate many manufacturers cycle the glow plugs at lower temperatures than is desired for efficient engine operation. More specifically, as the glow plug cycles between temperatures, it experiences internal stresses due to the differences in the thermal expansion rates between the differently composed layers. As the different layers expand and contract at different rates, stress may occur that may cause failure of the glow plug, commonly in the heating element of the glow plug.

Yet another drawback to the style of glow plugs 2 illustrated in FIG. 1 is that the electrical connection 9 between the conducting core 8 and the outer resistive layer 4 is in close proximity to the external surface of the glow plug 2 and may be subject to oxidation from the surrounding atmosphere during service. Sufficient oxidation at the electrical connection 9 can degrade the electrical connection 9 by the formation of an electrically insulating oxide layer, or the formation of a porous layer having an interfacial porosity, to the point where current can no longer pass between the conducting core 8 and the resistive layer 4, resulting in a failure of the glow plug to heat when an electrical current is applied.

Yet another drawback to the style of glow plugs 2 illustrated in FIG. 1 is that the inconsistencies in the layer thickness and geometry created by the casting process leads to inconsistent resistance between manufacturing lots. The cast layers are formed by a gradual buildup of material against either the mold or against a previously formed cast surface. Once the desired thickness is achieved, excess liquid casting slip is removed. The thickness is controlled primarily by casting time, but is also affected other factors including the rheological properties of the casting slip, the permeability of the mold, and the permeability of any previously cast layers. In addition, when the casting slip is removed, the newly cast surface remains wet for a short period of time, and this small amount of remaining liquid slip may form drips or runs that further contribute to non-uniform layer thickness. Any of these factors may cause small variations in the layer thickness and the uniformity of the thickness of the layers which result in variations in the electrical resistance of the glow plugs and variances in the heating profile of the glow plug.

It is therefore desirable to provide a heating element for use in glow plugs that overcomes the disadvantages of the prior art and in particular a heating element for glow plugs that has low internal thermal stresses, optimized geometrical shape for heating, increased longevity and durability, and precisely controllable and reproducible heating characteristics.

SUMMARY OF THE INVENTION

The present invention relates to heating elements and, in particular, to heating elements for glow plugs and gas igniters as well as the method of manufacturing thereof. The heating element generally includes a first layer formed from or acting as an electrically insulative material and a second layer formed out of a electrically conductive material that is molded around portions of the first layer. By varying the geometric profile of the first layer and the geometric profile of an injection die, the thickness of the conductive layer may be varied along the length as well as around the circumference of the heating element to provide a desirable heating profile for a specific application. The molded profile of the first layer and the profile of a die in which the electrically conductive layer is molded allows for these geometric profiles and variations in the heating profile that are not available with the slip casting method. Furthermore, by molding the electrically conductive layer as a single piece extending between a first electrical connection and a second electrical connection prevents many of the problems with the prior art methods by removing discrete interfaces between layers and eliminating the electrical interface.

The invention includes a method of forming a heating element including the steps of forming a first layer, placing the first layer in a die, and molding an electrically conductive layer around the first electrically insulative layer.

Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which:

FIG. 1 is a sectional view of a prior art slip casted heating element;

FIG. 2 is a sectional view of the present invention having the heating portion focused at a first end;

FIG. 3 is a sectional view of the present invention having an extended heating portion;

FIG. 4 is a sectional view of the present invention having a heating portion primarily focused at the first end;

FIG. 5 is a sectional view of the present invention having a heating portion primarily focused at the first end;

FIG. 6 is a diagram showing the method steps in forming a heating element;

FIG. 7 is a diagram of a first alternative method of forming a heating element

FIG. 8 is a diagram of a second alternative method of forming a heating element;

FIG. 9 is a cross-sectional view of the first layer along lines 9-9 in FIG. 8;

FIG. 10 is a cross-sectional view of the first layer in a die along lines 10-10 in FIG. 8; and

FIG. 11 is a cross-sectional view of the formed heating element along lines 11-11 in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention, as illustrated in FIGS. 2-5, is directed to a heating element 10 having an electrically insulative layer 20, formed from an electrically insulative material, and an electrically conductive layer 30, formed from an electrically conductive material. As illustrated in FIG. 2, the conductive material is attached to a first electrical contact 40 and a second electrical contact 42 which allow electrical current to flow through the conductive material to generate heat that is primarily focused where the thickness of the conductive layer 30 is at its thinnest point and has the smallest cross cross-sectional area. Although only FIG. 2 is illustrated with the electrical contacts, 40 and 42, the heating element 10 will be generally formed with electrical contacts, which may vary in size, shape and configuration. The heating element also may include a base portion 14 formed in a variety of configurations and shapes.

The insulative layer 20 further includes an outer surface 22 that creates a geometric profile that may vary in shape and diameter to create the desired heating profile. The insulative layer 20 generally includes a first end 26, a second end 28, and a center portion 27. A passage 24 extends from the first end 26 to the second end 28.

The insulative layer 20 is generally formed from an insulative material and can be made from any known electrical insulator by any known method. Such methods may include extrusion, molding, powder compaction, and other methods. Ceramic powders with gelling additives as well as certain thermoplastic materials can be formed and sintered to make good insulators. For example, the insulative material may be a material such as silicon nitride, silicon carbide, aluminum oxide, aluminum nitride, or other ceramic materials. This list of potential insulative elements in no way should limit the materials that may be used to form the insulator. The insulative material may be formed of any material that has good electrical insulation properties or is commonly used in heating elements as an insulative material. The insulative material may also comprise electrically conductive particles in a matrix of electrically insulating material, such as a composite of molybdenum disilicide and silicon nitride wherein the conducting molybdenum disilicide particles are present below the percolation threshold and are thus electrically isolated from one another.

By molding or by the other listed methods, the insulative layer 20 can be formed in a variety of shapes such as those in FIGS. 3 and 4, which were not previously possible using the slip casting method. It is preferable for the insulative layer 20 to be formed from a material that may be reliably molded into various shapes. FIGS. 4 and 5 illustrate profiles that have an outer diameter at a first end 26 that is greater than the outer diameter at a center portion 27. The second end 28 may also have a diameter that is greater than the center portion 27 and sometimes a diameter that is greater than the first end 26. As may be seen, the insulative layer 20 may be highly customized to provide specific heating profiles when combined with the conductive layer 30.

The conductive layer 30 is generally formed from a conductive material that allows electrical current to flow between a first electrical contact 40 and a second electrical contact 42. The conductive layer 30 generally forms an outer surface 12 of the heating element 10. By varying the thickness between the insulative outer surface 22 and the conductive outer surface 32, the heating profile may be adjusted. For example, as illustrated in FIG. 3, the center passage 24 is filled with conductive material of the conductive layer 30 and has a relatively large thickness which allows for less resistance and easier electrical current flow. However, the thickness of the conductive layer 30 in the heating portion 16 of the heating element 10 is much thinner which creates a greater resistance and increases the amount of heat output near the heating portion 16. Therefore, as current passes between the first and second electrical contacts 40 and 42, in the thinner areas of the conductive layer 30, the heating output will be the greatest. As illustrated in FIG. 2, the thin area is limited to only a portion of the tip of the heating element 10 thereby creating a heating profile that is primarily focused in the vicinity of the first end 26. The heating profile may be varied by changing the profile of either the insulative layer 20 or the conductive layer 30.

As illustrated in FIG. 3, the conductive layer 30 extends from an area proximate to the first end 26 of the insulative layer 20 towards the second end 28 along the center portion 27. This creates a heating profile that extends further along with a greater heating capacity than the heating element illustrated in FIG. 2.

The heating element 10 illustrated in FIG. 4 includes a heating portion that is primarily focused near the first end 26 of the insulative layer 20 where the thickness is much thinner than the thickness near the center portion 27 of the insulative layer. Therefore, the heating profile of the heating element 10 is primarily focused near the first end 26 of the insulator, however, the heating element does provide some heat along the center portion 27 of the insulator. FIG. 5 is a further variation of the heating element in FIG. 4 with the conductive layer 30 extending further along the center portion 27 of the insulator toward the second end 28.

As illustrated in step 301 of FIG. 8 and FIGS. 9-11, the heating element may include projections along the outer surface 22 that allows centering of the heating element in the die that receives the insulative layer 20 for overmolding with the conductive layer. These projections formed from the insulative layer 20 may also modify the heating profile by creating areas on the outer surface 12 of the heating element 10 that do not generate heat. Typically, at least three of these projections would be used to center the insulative portion within the die; however more or less may be used depending upon the geometric shape and the die. The conductive layer 30 may be formed from a variety of known conductive materials such as conductive materials formed from ceramic matter that are typically used in glow plugs today including molybdenum disilicide, titanium nitride, zirconium nitride and titanium boride. The conductive material may also comprise electrically insulating particles in a matrix of electrically conducting material, such as a composite of molybdenum disilicide and silicon nitride wherein the conducting molybdenum disilicide grains are present above the percolation threshold and thus form a continuous electrically conductive path through the material. The conductive layer may also comprise metals such as platinum, iridium, rhenium, palladium, rhodium, gold, copper, silver, tungsten and alloys of these to name a few. Generally the conductive layer 30 needs to be formed of a conductive material that allows for easy molding in a die. Any conductive material or resistive heating material currently in use with heating elements may be used.

The heating element 10 is generally formed by a method of first forming an insulative layer 20 illustrated as steps 101 in FIG. 6, 201 in FIG. 7, and 301 in FIG. 8. In a second step, an injection molding die 50 is provided having a geometric profile that will form the outer surface 12 of the heating element 10 and is illustrated as step 102 in FIG. 6, step 202 in FIG. 7, and step 302 in FIG. 8. Once the insulative layer 20 is formed to the desired geometric shape out of an insulative material such as by molding powder formation or other methods, the insulative layer 20 is inserted in an injection molded die 50 as shown in steps 103 in FIG. 6, 203 in FIG. 7, and 303 in FIG. 8. After the insulative layer 20 is placed into the die 50, the molten conductive material is forced into the die as illustrated in steps 104 in FIG. 6, 204 in FIG. 7, and 304 in FIG. 8. With the molten conductive material in the die and substantially filling the voids, the material is allowed to cool and harden as illustrated in steps 105 of FIG. 6 and 305 of FIG. 8. The formed heating element 10 is then removed from the die 50 as illustrated in step 106 of FIG. 6, 205 of FIG. 7, and 306 of FIG. 8. The heating element 10 is then sintered to form a monolithic material (not shown). In the method illustrated in FIG. 7, the excess material is removed in step 206.

It will be understood by one that is skilled in the art that ceramic materials are commonly formed by first forming an assembly of finely divided particles and subsequently firing the assembly to sinter the particles in to a monolithic article. Ceramic materials are commonly injection molded by mixing the particles with a thermoplastic medium or binder such as, but not limited to, wax or polyethylene or a blend of the two, and heating the resulting mixture so that the molten mixture is sufficiently fluid to fill a die cavity, and subsequently cooling the molded article to form a rigid part that can be removed from the die. Alternatively, non-thermoplastic binder medium such as agar/water may also be employed. The binder medium is then removed by a process commonly known as debinding, which may include solvent extraction and thermal debinding steps. The part is then fired under suitable conditions to sinter the particles together and form the final monolithic article.

A first layer can be formed from a material that is insulative or even non-insulative in some embodiments. By forming the first layer from a material that is later removable from the final cast part allows a method forming a heating element substantially following the above steps however, it would have an additional step (not shown) of removing the first layer material from within the conductive layer 30. Removal of the first layer would create a substantially air core to the conductor which would act as an insulator. By having a hollow core all differences in thermal expansion are eliminated and may provide longer life to the heating element. Therefore the first layer may use any material that is known in the casting or molding process to be able to be later removed or destroyed during the casting process. To provide rigidity to the conductive layer, after the first layer is removed, an insulative or rigid layer may be added to fill the pockets within the conductive layer, such as an insulator that is not conducive to the overmolding process.

As is illustrated in FIGS. 6 and 7, the second end 28 of the insulative material 20 may engage the inner surface of the die to hold the insulative layer 20 in place within the cavity of the die 50. This ensures proper placement within the die 50 so that as the molten conductive layer 30 flows and is forced into the die the desired profile is created and the insulative layer 20 does not move. However, in some embodiments it may be desirable to have projections, as illustrated in FIG. 9, on the insulative layer 20 so that the projections engage the die as shown in step 303 in FIG. 8 and FIG. 10. These projections ensure that the insulative layer 20 stays in place during the molding process by providing two areas of contact with the die that are removed from each other.

By forming a first layer through extrusion molding or powder compaction methods similar to that currently used to form spark plug insulators, the first layer can be created with a specific geometric profile. The first layer can be formed from an insulative material for use in conjunction with the conductive layer, or from a material that is easily removable once the conductive layer is overmolded on the first layer. When this geometric profile the first layer is combined with the geometric profile of the conductive layer 30, a heating profile may be created for the heating element 10 that allows hot and cold spots and even areas that gradually change on the heating element, both around the circumference as well as along the length. Therefore, if needed, a heating profile can be created that has, for example, a hot spot on half of the circumference of the heating element 10 and removed from the first end 26 of the insulative layer 20 so that on the heating element 10 the tip of the heating element as well as at least half of the circumference and the portion toward the second end may be cooler than the desired hot spot. These types of heating profiles were previously unobtainable by the prior art methods of creating heating elements.

The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims. 

1. A heating element comprising: an electrically conductive layer; and an electrically insulative layer, and wherein said electrically conductive layer includes a portion that is completely formed within said insulative layer, and said insulative layer includes a portion that is completely formed within said conductive layer.
 2. The heating element of claim 1 wherein said heating element has a first outer surface defined by said electrically conductive layer and wherein said electrically insulative layer has an outer insulative surface, said conductive layer having a thickness between said outer insulative surface and said outer surface.
 3. The heating element of claim 2 wherein said thickness is variable with areas of less thickness having greater electrical resistance.
 4. The heating element of claim 3 wherein said thickness varies along the length of the heating element.
 5. The heating element of claim 3 wherein said thickness varies about the circumference of the heating element.
 6. The heating element of claim 1 wherein said heating element has a length and said electrically insulative layer includes an outer insulative surface and wherein said outer insulative layer has a diameter that varies along said length.
 7. A heating element comprising: a base portion having a first diameter; and a heating portion formed at an end of the base portion, said heating portion having a second diameter and wherein said first diameter is greater than said second diameter, and wherein said heating portion includes an electrically insulative layer and an electrically conductive layer, said conductive layer having a first thickness and a second thickness, said second thickness being greater than said first thickness.
 8. The heating element of claim 7 wherein said base portion includes said second thickness and said heating portion includes said first thickness.
 9. A heating element comprising: a first layer having a first geometric profile; and a second layer molded over said first layer, said second layer having an outer surface forming a second geometric profile and wherein said first and second geometric profiles create a variable thickness in the second layer.
 10. The heating element of claim 9 wherein said variable thickness creates areas of higher electrical resistance and areas of lower electrical resistance.
 11. The heating element of claim 10 wherein said first and second geometric profiles are designed to optimize a heating profile.
 12. The heating element of claim 9 wherein said first layer is an electrical insulator and said second layer is an electrically conducting material.
 13. The heating element of claim 12 wherein said electrical insulative layer and said electrically conductive layer have approximately the same thermal expansion characteristics.
 14. The heating element of claim 13 wherein said electrically insulative layer is formed from polystyrene.
 15. A heating element having a length comprising: a first layer including at least two projections; and a second layer substantially surrounding said first layer for at least a portion of the length of the heating element, wherein said second layer and said at least two projections form an outer surface.
 16. The heating element of claim 15 wherein said first layer includes at least three projections.
 17. A method of forming a heating element comprising: forming a first layer having at least three projections; inserting said first layer into a die so that said projections engage the die; and forming an electrically conductive layer at least partially around said first layer.
 18. The method of claim 17 wherein said step of forming an electrically conductive layer includes the step of injecting a molten conductive material into said die.
 19. The heating element of claim 17 wherein said first layer has a center passage and an outer surface and wherein said second layer is disposed within said center passage and surrounds at least a portion of said outer surface.
 20. The heating element of claim 19 wherein said second layer is formed without joints or interfaces.
 21. The heating element of claim 19 wherein said second material is formed as a single piece.
 22. A heating element having: a first layer having a first end, a second end and a center portion between said first and second ends, and wherein said first and second ends have a diameter that is greater than the diameter of the center portion; and a second layer.
 23. The heating element of claim 22 wherein said second layer is electrically conductive to provide resistance heating.
 24. The heating element of claim 22 wherein said first layer is an electrical insulator.
 25. The heating element of claim 22 wherein said second layer is molded around said electrically insulative layer.
 26. The heating element of claim 22 wherein said second layer has an outer surface and wherein the diameter of said outer surface proximate to said first end of said first layer is less than the diameter of the outer surface proximate to the center portion of said first layer.
 27. A method of forming a heating element comprising: molding a core from a first material; overmolding an electrically conductive material on said core; and removing said core from said conductive material.
 28. The method of claim 27 further including the step of adding an electrically insulative material to said conductive material after said step of removing said core. 